This invention relates to a method of lubricating silicon nitride ceramics and the like.
Advanced ceramics offer great potential for future engineering applications. Their unique blend of strength, wear and corrosion resistance and light weight enables technologies that were not possible otherwise. Technologies such as low heat rejection engines, advanced gas turbines, environmentally compatible fuel efficient diesel engines, and space structures all depend on the availability of such advanced materials. The brittleness of ceramics, however, causes concern in terms of reliability and durability, especially since effective reliable lubrication technology of ceramics does not currently exist. This limits the load carrying capacity of the ceramics and durability of the ceramic component.
Silicon nitride, e.g., Si.sub.3 N.sub.4, is the most promising ceramic for future applications in bearings, tools and engine components. The ability to lubricate these materials under high stress (boundary) conditions will become critical as the technology continues to mature.
In general, conventional lubricants rely on special chemical compounds to reduce friction and wear under high stress (boundary) conditions. One of the key concepts in boundary lubrication is that chemical reactions occur with the surface to produce a protective film. This is described in an article by S. M. Hsu, "Boundary Lubrication of Materials," MRS Bulletin, October 1991, pp. 54-58.
Many of the concepts of boundary lubrication on metal surfaces rely on chemical reaction with the metal surface and/or catalysis by the metal. This is easily understood because most metal system are chemically reactive. Current lubricants for iron-based systems are based on the reactions between P, S and Cl with iron. The resultant protective boundary lubricating films are rich in iron-organo-metallic compounds containing P, S and Cl.
Ceramic materials, however, contain substantially no iron (less than 0.2%). Further, ceramic materials are generally not considered to be reactive, relative to iron-based systems. In fact, ceramics are used in many applications because they are considered to be chemically inert and are thus, useful in high temperature and corrosive environments. Consequently, iron chemistry with conventional antiwear compounds would not be expected to be applicable to boundary layer lubrication of ceramic materials, such as silicon nitride.
Furthermore, one of the key roles of boundary lubricants in metal systems is to serve as a barrier film separating the two surfaces to prevent adhesion--a dominant wear mechanism in metal systems. Ceramic systems, on the other hand, are dominated by brittle fracture as a dominant wear mechanism. The mechanical strength, surface morphology, and film thickness for ceramics therefore are different from metal systems. To minimize fracture in ceramics, the dominant role of the lubricating films is to redistribute the asperity stresses in a contact. Therefore, the film should be thicker, stronger in shear strength, and faster reacting than films for metal systems. The distinction in the nature of wear mechanisms between metal systems and ceramic systems further supports the expectation that conventional concepts of boundary lubrication for metal systems would not be applicable to ceramic systems.
Prior art addressing the issue of Si.sub.3 N.sub.4 lubrication is sparse and inconclusive. Willermet used several material pairs (cast iron/cast iron, steel/steel, Si.sub.3 N.sub.4) and a pair of formulated lubricants to show that an oil soluble molybdenum containing compound reduced friction in an LFW-1 bench test for all material pairs, P. Willermet, "An Evaluation of Several Metals and Ceramics in Lubricated Sliding," ASLE Transactions, 30, 1, pp. 128-130, 1987. The oils were both SAE 30 with Zn dithiophosphate antiwear additive and an overbased detergent package containing Mg and boron. No wear data were provided. Klaus used a Ball-on-Three-Flat (BTF) wear tester to examine the performance of a SAE 5W-30 commercial oil on Si.sub.3 N.sub.4 specimens. He found that at 40 kg applied load, the formulated oil had lower wear than a straight mineral oil. It was not determined which components in the commercial oil were responsible for the lower wear, E. E. Klaus et al., "Lubricated Wear of Silicon Nitride," Lubrication Engineering, 47, pp. 679-687, 1991.
As to oxygen-containing compounds, Jahanmir, "Friction and Wear of Silicon Nitride Lubricated by Humid Air, Water, Hexadecane and Hexadecane+0.5 Percent Stearic Acid", STLE Transaction, 31, pp. 32-43 (1988), indicated that 0.5% stearic acid in hexadecane under low load reduced the wear of Si.sub.3 N.sub.4, however, no experimental evidence was offered to link direct chemical reaction with Si.sub.3 N.sub.4. Indeed, Habeeb, I. Mech. E C132/87, p. 555-564 (1987), found that 0.4% lauric acid in a 150 neutral base oil increased the wear of Si.sub.3 N.sub.4. Tsunai "Tribochemical wear of silicon Nitride in Water, n-Alcohols and Their Mixtures", Wear of Materials, p. 369-374 (1989) suggested that alcohols might react with Si.sub.3 N.sub.4 and SiC for lubrication, however, experiments were conducted only at unrealistically low sliding speeds of 0.002 m/s, and the chemicals were limited to linear alcohols of carbon number 10 or less. The proposed mechanism was based on a reaction study of C.sub.1 to C.sub.3 alcohols with Si.sub.3 N.sub.4 performed by Hattori, "Reactions of Silicon Nitride with Alcohols", the 56th Annual Meeting of Chem. Soc. Japan, Abstract, Vol. I, Tokyo, p. 790, April 1988, who did not present clear experimental evidence to support his proposal. Tsunai used Hattori's speculation on reaction mechanism even though Tsunai was unable to find the same reaction products in his wear tests. See also Hibi "Friction and Wear of Silicon Nitride in Water, n-Alcohols, Water-Methanol and Water-Glycol, "Bulletin of Mechanical Engineering Laboratory", No. 53 (1990).
Attention is also directed to Gates et al., "Effect of Selected Chemical Compounds on the Lubrication of Silicon Nitride," Tribology Transactions, 34, 3, pp. 417-425 (1991), which discloses that certain specific compounds show a lubrication effect upon silicon nitride.